The buttons are active low, meaning the input will be connected to ground when the button is
activated. The buttons have no external pull-up resistor, so to use the buttons the
P0.13–P0.16 pins must be configured as an input with an internal pull-up
resistor.

The LEDs are active low, meaning that writing a logical zero ('0') to the output pin will
illuminate the LED.

Figure 2. Button and LED configuration

I/O expander for buttons and LEDs
The nRF52 DK has an I/O expander to avoid conflicts with boards that follow the Arduino standard, the onboard GPIOs for the buttons and LEDs would otherwise possibly conflict with such boards.